1. Field of the Invention
This invention relates to a phenolic resin composition useful for preparing reinforced composites. The present invention particularly relates to a resole-type phenolic, resin containing a sulfite or bisulfite compound which is useful for preparing reinforced composites. The resole resins of the present invention possess superior water dilutability and storage stability, reduced volatile emissions upon curing, and excellent compressive strengths and flexural strength when used to make reinforced composites. The reinforced composites may use inorganic and/or organic fibers, which may be chopped, non-woven or woven, and honeycombed materials as the reinforcing materials, e.g. to form prepregs which are laminated and cured, and cured honeycomb composites for various conventional applications, particularly those requiring excellent flame and smoke properties.
2. Description of the Prior Art
Phenolic resins have long been employed as binders for use with glass fibers in the preparation of fiber reinforced composites. Glass fibers which may be chopped, non-woven or woven are coated or saturated with the aqueous binder solution, usually by dipping or spraying, and passed through an oven where they are compressed to the desired thickness and density, and then permanently fixed by heat setting or curing the resin binder. Phenolics are preferred over many other polymer systems, such as polyesters and epoxies, due to their well known excellent resistance to flame and smoke generation. Thus, their use in the aerospace, ship industry, mass transit, and other applications requiring resistance to fire and smoke generation is expected to grow. However, environmental considerations have directed the composite industry away from the solvent-based phenolic resins. Efforts to employ water-based phenolics have previously resulted in problems with reduced strengths, e.g., reduce compressive strengths, and poor water dilutability during storage.
The properties desired of binder compositions depend to a large extent on the properties of the basic resin. A good binder composition should above all be easily applied and capable of covering and/or bonding the reinforcing components, e.g. fibers and honeycombs, and at the same time it should cause little pollution. Further, the resin should have good long term stability and a high degree of dilutability with water. Since the concept of dilutability is particularly important, it will be defined for the purposes of the present invention as follows: The water dilutability of a resin solution is the volume of de-ionized water which can be added at a given temperature to one unit volume of this solution without producing any permanent perturbation, i.e., haze, clouding or precipitation.
Of particular interest are high efficiency single phase aqueous phenol-formaldehyde resins which have low free phenol and low free formaldehyde. Such resins retain a high percentage of the organic moiety when the resin is cured. However, free phenol and free formaldehyde volatilize in the curing of the resin causing pollution considerations and also reducing the efficiency and performance of the resin in various bonding applications. Accordingly, it is necessary that the resin should be as free as possible from any unconverted starting materials or residues thereof while preserving its useful qualities. The risk of atmospheric pollution is in fact mainly due to the presence of volatile monomers. These consist, for example, the starting materials required for producing the resin, e.g. formaldehyde and phenol, which have been left unconverted by the reaction or regenerated in the course of binding the fibers or subsequently.
The utilization of sulfite or bisulfite compounds in phenol-formaldehyde condensation products is known. For example, Higginbottom in U.S. Pat. No. 3,956,205 disclose, s the use of sodium sulfite as a formaldehyde scavenger in such condensation products. Similarly, Meyer in U.S. Pat. No. 4,264,760 discloses the addition of sulfur-containing compounds, e.g., sulfite and disulfites, in sufficient quantities to bind the free formaldehyde in resins, including phenol-formaldehyde resins. Alkali metal sulfites and bisulfites are also used as catalysts in the preparation of phenol-aldehyde resins. See Dietrick, et al. in U.S. Pat. No. 3,328,354 and Hood et al. in U.S. Pat. No. 4,608,408 (phenol-resorcinol-formaldehyde resins), and Daubach, et al. in U.S. Pat. No. 3,872,056 (phenol-formaldehyde condensates).
Sulfites and/or bisulfites have also been added to phenol-formaldehyde resins or formed in situ following an alkali catalyzed reaction. According to the process of Komarck et al. in U.S. Pat. No. 3,065,039 for preparing tanning agents, phenols are reacted with formaldehyde in an alkaline medium (e.g., by adding sodium hydroxide) to form a low molecular weight resin containing methylol groups. Sulfur dioxide is then introduced into the resin by blowing or pouting, until a permanent neutral reaction is obtained. Sodium bisulfite is formed in situ if sodium hydroxide was used in the alkaline medium. The sodium sulfite solubilizes the resin. In Baxter, U.S. Pat. No. 3,108,990, sodium sulfite and/or sodium bisulfite are used as antioxidants to prevent scumming and improve storage-life of highly alkaline (pH of 10 to 14), high molecular weight, phenol-formaldehyde resins. Baxter observed that the addition of small quantities of sodium sulfite to a phenolic resin did not deleteriously affect the resin or its resin adhesive properties and water solubility, particularly for use in plywood production.
The present invention is based on the discovery that the addition of a proper amount of a sulfite or bisulfite compound(s) to a phenolic resole resin result in unexpected and large increases in compressive strengths and flexural strengths measured on glass fiber reinforced laminates prepared using the resins of the present invention. The modified resins also possess increased water dilutability and storage life and dramatic reductions in organic volatile emissions during cure.